Recently electronic appliances are promoted in the trend of downsizing to be used as portable units or to concentrate multiple functions. The variable resistors used in such small-sized electronic appliances are also demanded to be reduced in size.
Moreover, along with wide distribution of IC, microcomputers and the like, variable resistors are more often used as means for adjustment of direct-current voltage.
In variable resistors in such conditions, troubles due to electromigration are likely to occur. To prevent such troubles, the following measures have been attempted. To begin with, electromigration is a phenomenon of ions moved by an electric field. For example, in an electronic circuit, ion components generated from the conductor depending on the environments of use move toward other conductor having a different potential, and, as a result, a defective portion of electric insulation is formed.
A prior art of this kind in a small-sized rotary manipulation type variable resistor is described while referring to the drawings.
FIG. 8 is a side sectional view of a conventional rotary manipulation type variable resistor, FIG. 9 is a perspective appearance of a resistance substrate shown in FIG. 8, and FIG. 10 is a perspective appearance of a current collector shown in FIG. 8. In FIG. 8, a conventional rotary manipulation type variable resistor comprises a case 1, a resistance substrate 3 having a resistance element layer 2, a current collector 4, a slider 7, and a manipulation knob 6. The case 1 is made of an insulating resin, and is formed like a box, having an opening portion in the upper side and a small penetration hole 1A in the center.
The resistance substrate 3 is a hard electric insulating substrate. The resistance element layer 2 is formed on the surface of the resistance substrate 3 by printing, and is formed like a horseshoe. The resistance substrate 3 having the resistance element layer 2 has a circular hole 3A formed in the center of the resistance element layer 2. The current collector 4 is a circular metal plate, and a small circular hole 4A is formed in its center. The current collector 4 is overlaid and disposed beneath the resistance substrate 3 so as to be concentric with the circular hole 3A.
Connection portions 2A, 2B are disposed at both ends of the horseshoe resistance element layer 2. The connection portions 2A, 2B are formed by printing by using silver ink. Terminals 5A, 5B are crimped to the connection portions 2A, 2B. A terminal 4B is integrally formed on the current collector 4. Each one of the terminals 5A, 5B and terminal 4B is disposed so as to project outward of the case 1.
The case 1 is formed and fabricated in a state of inserting the resistance substrate 3 having the resistance element layer 2 and the current collector 4, and at the same time the small penetration hole 1A is formed in the center. Thus, the resistance substrate 3 and current collector 4 are fixed, and the case 1 having the small penetration hole 1A is formed.
The manipulation knob 6 is disposed to cover the opening of the case 1, and it is made of insulating resin and is designed to be manipulated by rotation. The manipulation knob 6 has an anti-slip protrusion 6A and a shaft 6B formed in the lower part of the center. The slider 7 made of an elastic thin metal plate is held at the lower side of the flat plate of the manipulation knob 6. The anti-slip protrusion 6A and shaft 6B are passed in the small penetration hole 1A, so that the manipulation knob 6 is held rotatably in the case 1. An elastic contact point 7A elastically contacts with the resistance element layer 2, and an elastic contact point 7B elastically contacts with the current collector 4. To stabilize the contact, silver plating is applied on the contact surface of the current collector 4 at the contact portion of two metal plates. Further, to prevent damage of surface due to seizure by rotation and sliding, contact point grease of low viscosity is applied on the contact surface of the current collector 4.
In thus constituted rotary manipulation type variable resistor, by applying a force on the outer circumference 6C of the manipulation knob 6 in the tangential direction, the manipulation knob 6 is rotated, and the elastic contact point 7A of the slider 7 elastically slides on the resistance element layer 2, and the elastic contact point 7B elastically slides on the current collector 4. Thus, the resistance value between the terminal 5A and terminal 4B, or between the terminal 5B and terminal 4B is changed.
In the rotary manipulation type variable resistor having such constitution, electromigration of silver ions may occur. The location is the point of occurrence of potential difference due to presence of silver. Positions corresponding to this condition are (A) and (B):
(A) The area between connection portion 2A and connection portion 2B at both ends of the resistance element layer 2 in which silver ink is printed on the resistance substrate 3.
(B) The area between the silver plated current collector 4 and the resistance element layer 2.
To prevent electromigration in these areas, a sufficiently long spacing is provided between the connection portion 2A and connection portion 2B at both ends of the resistance element layer 2. Moreover, as the current collector 4 is disposed beneath the resistance substrate 3, a spacing corresponding to the plate thickness of the resistance substrate 3 is provided between the current collector 4 and resistance element layer 2. Therefore, electromigration has been prevented in these areas (A) and (B).
However, in such conventional rotary manipulation type variable resistor, the current collector 4 and resistance substrate 3 are composed by using mutually different materials. Accordingly, by using an insulating resin, when forming and processing the case 1 while inserting the current collector 4 and resistance substrate 3, the manufacturing process was complicated. It was also required to apply a contact grease on the contact surfaces of the current collector 4 and elastic contact point 7B. Still more, the contact grease may be adhered to the resistance element layer 2, and the resistance element layer 2 may be damaged by the elastic contact point 7A.
It is hence an object of the invention to present a rotary manipulation type variable resistor free from occurrence of migration even in the state of use in direct current, easy in manufacturing process, not requiring application of contact grease on contact surface of elastic contact point, and capable of reducing in size.